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31 January 2020

Friday fold: tension gashes near Sunflower, Arizona

Reader John Christian shared these folds with me via email last week. They are quartz veins in slightly metamorphosed Precambrian igneous rocks found near Sunflower, AZ in the Mazatzal Mountains. The second photo is a close-up shot of the curviest, cleanest batch of folds from the first shot. These are beautiful examples of folds in similarly oriented quartz veins; we call them “en echelon” for the way you have a …


21 November 2018

Brittle overprinting ductile in the South Mountains metamorphic core complex, Phoenix, Arizona

Today, we take a look at the structural geology that reveals the deformation evolution (first ductile, then brittle) of the South Mountains metamorphic core complex, south of Phoenix, Arizona. Expect lots of photos of smeared-out rocks, broken by faults.


17 November 2018

Flow banding in pseudotachylyte, South Mountains, Arizona

Watch the flow of frictional melt in a “fossil earthquake,” frozen in time atop the South Mountains metamorphic core complex in Phoenix, Arizona.


22 August 2018

Q&A, episode 5

A reader asks: “What is foliation and what makes it so important to the structure of rock?”
Callan answers with a lot of images of beautifully foliated rocks.


25 May 2018

Friday fold: 3D model of intense folding

This is a block of rock I found in the rock garden at Northern Arizona University a few weeks ago: [sketchfab id=”16a73845eaf44a23b28b2f1bb3921ba9″ start=”0″ spin=”” controls=”0″] It’s intensely folded. Not sure what kind of rock it is, but it was quite dense.


12 January 2018

Friday folds: Hayden Butte (“A Mountain”), Tempe

In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has …


11 January 2018

Landslide breccias in Papago Park

A visit to Papago Park, north of Tempe, Arizona, reveals hanging wall rocks from the South Mountain detachment fault, a long way from South Mountain. Also, feast your eyes on these gorgeous red sedimentary breccias, interpreted to be landslide deposits from the Neogene.


10 January 2018

SGTF field trip

A virtual field trip to the Phoenix Mountains and South Mountains of Arizona, along with participants in the 2018 Structural Geology and Tectonics Forum.


10 February 2017

Friday fold: the Grand Canyon

Deep in the Grand Canyon of Arizona, Steve Mirsky found a Friday fold to share with you.


22 April 2013

Monday Geology Picture(s): Petrified Trees

My friend Ray recently visited Petrified Forest National Park in Arizona and was kind enough to send me some of his gorgeous pictures of the landscape, including the petrified trees. Ray says that these pictures were all taken along the Blue Mesa Loop. These fossilized trees are about 225 million years old! They are weathering out of the Chinle Formation, a beautiful Triassic sedimentary unit. I’ve never visited this park, …